The present invention relates generally to the material separation art and, more particularly, to a particle separation system, including an improved electrostatic separation apparatus, and a related method for electrostatically separating two species of particles present in a particle mixture.
Various types of apparatus for removing particles from a dry fluid flow using electrostatic separation techniques are well known in the art. An early example of such an apparatus is shown in U.S. Pat. No. 3,493,109 to Carta et al., the operation and limitations of which are described in detail in commonly assigned U.S. Pat. No. 5,755,333, issued May 26, 1998. Generally speaking, the Carta et al. patent relies upon turbulent flow and particle-wall contact in the separation chamber to electrostatically charge the particles. The particles are then drawn from the flow by opposed electrically conductive plates having opposite polarities.
While the apparatus proposed in the Carta et al. reference is somewhat effective for separating particles having a selected charge from a particle mixture, several significant limitations remain. For instance, no effective means is disclosed to ensure that once separated, the selected particles will be directed to the appropriate collection device. To the contrary, the apparatus disclosed in the Carta et al. patent promotes turbulent flow in the separation chamber, which can allow deleterious re-mixing of the particles to occur after separation. As should be appreciated, this reduces efficiency to the point that several cycles or passes through the apparatus may be required to achieve separation. In addition to reducing efficiency, multiple passes significantly increase the particle abrasion to which the wall of the apparatus is subjected thereby reducing the service life of the separator.
In an effort to overcome this shortcoming, commonly assigned U.S. patent application Ser. No. 08/726,255, entitled xe2x80x9cApparatus and Method for Triboelectrostatic Separation,xe2x80x9d proposes an improved apparatus for separating two species of particles from a particle mixture with greater efficiency and effectiveness by using a curtain gas flow to carry the selected particles drawn from the mixture to a collector for recovery. Similar to the apparatus proposed in the Carta et al. patent, separation is effected through the use of oppositely charged conductor plates connected to a variable voltage source. The charged plates attract oppositely charged particles away from the mixture and towards the sidewalls of the separation chamber. The curtain gas flow (which is initially devoid of particles) is then introduced into the separation chamber to provide the cleaning action necessary to remove or sweep the particles from the plates for recovery.
While this apparatus is effective for separating two particle species from a particle mixture, it should be appreciated that further improvements in separation effectiveness and operational efficiency are still possible. More specifically, there is a need for an electrostatic separation apparatus that: (1) reduces turbulence in the separation chamber created by the mixing of the particle and curtain gas flows to ensure that more selected particles are separated from the particle mixture and collected for recovery; (2) enhances particle separation efficiency by permitting adjustments to be made to the length of the electric field during operation; and/or (3) includes serial separators in each apparatus to ensure that the particle species are fully separated in a single pass. Additionally, including a plurality of the electrostatic separation apparatus meeting these criteria in a system fed by a single distributor would improve operating efficiency and greatly increase the throughput or volume of the particle mixture that can be processed in a given time period.
Accordingly, it is a primary object of the present invention to provide an improved apparatus for electrostatically separating two species of particles from a particle mixture that overcomes the above-identified limitations and shortcomings of the prior art.
Another object of the present invention is to provide a particle separation system including a single distributor for simultaneously supplying the particle mixture to a plurality of individual electrostatic separation apparatus, whereby two distinct and substantially pure particle species are recovered from each apparatus.
Still another object of the present invention is to provide an electrostatic separation apparatus having multiple separators in series for removing and collecting selected charged particles from a particle mixture, whereby after passing the particle mixture through said multiple separators, two distinct and substantially pure species of particles are fully recovered.
Yet another object of the present invention is to provide an electrostatic separation apparatus that includes flow straighteners for both the particle mixture and curtain gas flows to reduce turbulence in the separation chamber and improve separation efficiency.
A further object of the present invention is to provide an electrostatic separation apparatus having at least one separation chamber wherein the length of the electric field zone is selectively adjustable for varying the quantity and quality of the selected charged particles that are drawn from the particle mixture.
Additional objects, advantages and other novel features of the invention will be set forth in part in the description that follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned with the practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention as described herein, an improved electrostatic separation apparatus is disclosed and use of this apparatus as part of an overall particle separation system is described. In the broadest aspects of the invention, the improved electrostatic separation apparatus includes at least one separator having an electrostatic separation chamber. Preferably, the separator is cylindrical and has a first inlet that receives a particle mixture entrained in a dry driving fluid, such as air. The inlet is designed to create a spiraling, turbulent flow that assists in generating a differential charge on the two constituent species of particles present in the mixture prior to delivery to the separation chamber.
The separation chamber includes an electric field zone for drawing particles having a selected charge from the mixture. Specifically, a concentric electrode and conductor at different electric potentials are provided in the separation chamber to create a non-uniform electric field zone. A variable voltage source may be connected to the electrode and conductor to control the polarity and magnitude of the voltage applied to each. Alternatively, the potential difference may be created by connecting the variable voltage source to the conductor and grounding the electrode. Depending on the charge and amount of voltage applied to the conductor and the electrode, the difference in potential across the non-uniform electric field creates an electric field force vector that draws particles having a selected charge away from the particle mixture flow.
To remove or sweep the selected charged particles drawn from the particle mixture in the electric field zone, a flow of curtain gas is provided. Specifically, a curtain gas chamber concentric with the first inlet includes a first curtain gas inlet for supplying a first curtain gas flow to the separation chamber. The curtain gas entrains the particles drawn from the mixture and carries them to a first collector for recovery.
In accordance with an important aspect of the present invention, improved separation efficiency is provided by substantially eliminating turbulent flow in the separation chamber. Specifically, the first inlet includes flow vanes to straighten and smooth the particle mixture flow prior to entering the separation chamber. Likewise, flow straighteners near the exit of the curtain gas chamber ensure that the curtain gas entering the separation chamber is smooth and substantially parallel to the particle mixture flow. Advantageously, the smooth, parallel flows of the particle mixture and curtain gas keep turbulence at a minimum, which improves separation efficiency. Additionally, the flows of the curtain gas and the particle mixture are preferably metered to ensure that both are at substantially the same velocities upon entering the separation chamber. As should be appreciated, the parallel, smooth flows and matched velocities prevent deleterious re-mixing of the separated particle species entrained in the curtain gas and those remaining in the particle flow after separation. Moreover, this flow pattern ensures that the curtain gas flow remains effective in carrying the selected charged particles drawn from the particle mixture in the first electric field zone to the first collector for recovery.
The first collector includes a first discharge outlet for discharging the recovered selected charged particles to a first collection bin. A transition outlet downstream of the first separation chamber delivers the particle flow remaining after passing through the first separator to a next-in-line separator. Alternatively, the remaining particle flow may be delivered directly to a second collection bin if further separation is unnecessary or undesired.
As noted above, the electric field zone is created between the conductor and the electrode by supplying a different electric potential to each to create a non-uniform electric field that serves to draw particles having a selected charge from the particle mixture. In the preferred embodiment, the conductor is cylindrical and is positioned or embedded in an outer wall of the separation chamber. The electrode is positioned at the center and may extend through the separation chamber only or, alternatively, from the first inlet, through the separation chamber, and into the discharge outlet. However, portions of the electrode outside of the separation chamber are preferably coated with a non-conductive material to prevent deleterious sparking between the walls of the inlet or outlet and the electrode.
In accordance with an important aspect of the present invention, the length of the electric field zone in the separation chamber is selectively adjustable, even during operation of the separation apparatus. This is accomplished by adjusting the relative position of an inner partition projecting into the separation chamber that actually defines the transition outlet and inlet for the next-in-line separator. By moving this inner partition upwardly into the separation chamber, the amount of the particle mixture flow that is exposed to the first electric field zone is reduced. This will of course result in a reduced draw of selected charged particles, unless the strength of the electric field force vector in the electric field zone is increased by increasing the potential difference between the electrode and conductor, such as by adjusting the magnitude of the charge supplied by the variable voltage source. By selectively adjusting the length and the strength of the electric field zone, optimum conditions can be easily achieved during operation to improve separation efficiency. Furthermore, as will be more readily understood from reviewing the disclosure which follows, by adjusting the electric field zone(s) in downstream separator(s), it is possible to further optimize the total amount of selected charged particles separated from the particle mixture by the apparatus and the number of stages required to acquire two substantially pure particle species.
In accordance with a more specific object of the present invention, a plurality of separators of the type described above may be included in a single electrostatic separation cell. More specifically, the particle flow remaining after the selected charged particles are recovered in a first separator is delivered through the transition outlet to a second separator. The second separator includes a second inlet for receiving the remaining particle flow, which may still contain selected charged particles not separated and carried away as the flow passed through the first separator.
To ensure that the remaining particle flow is adequately charged prior to entering the second separator, the discharge outlet defined by the partition tapers inwardly toward the second inlet to create a turbulent flow. Additionally, a spiral insert or the like may be provided in the second inlet for assisting in recreating turbulent flow. This promotes particle-particle and particle-wall contact which charges the particles. However, prior to exiting the second inlet, the particle flow is passed through a second flow vane for straightening. This ensures that the remaining particle flow is substantially parallel to a vertical axis of symmetry of the second inlet upon entering the second separation chamber.
Upon exiting the second inlet, the particle flow enters a second separation chamber. This second separation chamber includes a second electric field zone for drawing any remaining selected charged particles from the remaining particle flow. The second separation chamber may be substantially identical in construction to the first separation chamber, such that the second electric field zone is created by an electrode and a concentric conductor held at different electric potentials. The electrode in the second separation chamber may be an extension of the electrode in the first chamber, or alternatively, can be a separate segment. In the latter case, a separate variable voltage source can be connected between each segmented electrode and conductor. This arrangement allows for the potential difference across the electric field zone in each separator to be selectively adjusted to improve particle separation.
To carry away the selected charged particles drawn from the remaining particle flow, a second curtain gas flow is provided. The second curtain gas flows from a second curtain gas chamber concentric with the second inlet. Flow straighteners positioned near the exit of the second curtain gas chamber ensure that the second curtain gas flow remains substantially parallel to a vertical axis of the separation chamber prior to entering the second separator. Similar to the operation of the first separator, the second curtain gas flow entrains and carries the selected charged particles removed by the second electric field zone to a second collector for recovery. As the species of particles collected in the second separator are the same species that were removed in the first separator, the second collector is connected to a manifold in fluid communication for delivering the particles collected to the first collection bin.
As should be appreciated, a third separator similar in operation to the first and second separators described above may form a part of the electrostatic separation apparatus of the present invention. Furthermore, each electrostatic separation apparatus may include more than three separators arranged serially, depending on the types of particles being separated, the flow rate, the differential charges created on the particles, and other parameters. Preferably, the last separator in the separation apparatus includes a terminal outlet for discharging the particle flow remaining after the selected charged particles are fully removed to a second collection bin, which of course should be a substantially pure, commercially acceptable product consisting of only a single species of particles. Thus, after processing the particle mixture through the separation apparatus, the result is a single species of particles held in the first collection bin and a second species of particles held in the second collection bin.
In accordance with another more specific aspect of the present invention, a plurality of the electrostatic separation apparatus described above may be fed by a single distributor. Advantageously, this allows for simultaneous separation to occur to further enhance operational efficiency. In the preferred embodiment, the distributor is frusto-conical in shape and includes a plurality of discharge ports each corresponding to the first inlet of a electrostatic separation apparatus positioned adjacent to the distributor. Ideally, the particle mixture is tangentially introduced from the distributor into the separation apparatus to initiate turbulent flow to induce particle-particle and particle-side wall contact which creates the differential charging of the two particle species that is beneficial for particle separation.
Yet another more specific aspect of the invention is to include the plurality of electrostatic separation apparatus and the distributor in a particle separation system. In the preferred embodiment, the system includes a driving fluid source, such as a forced draft fan for blowing ambient air into a feed line supplying the distributor. The driving fluid source is positioned upstream of the distributor and an induction source, such as an induced draft fan, is provided downstream of the first and second collection bins to draw the driving fluid through the system. The action of the induced draft fan may also provide the source for the curtain gas flow for each separation stage. A feeder is provided in fluid communication with the forced draft fan for supplying the particle mixture to the distributor, which as described above is preferably frusto-conical in shape. The distributor assists in charging the particle mixture and then supplies a portion of the mixture to each of a plurality of electrostatic separation apparatus of the type described above including at least one, and preferably a plurality of, separators each having a collector. Each collector includes a discharge outlet for discharging the selected charged particles to a first collection bin, while the particle flow exiting the last-in-line separator is discharged through a terminal discharge outlet feeding a second collection bin. The first and second collection bins in turn discharge the particle flow to separate first and second collection devices, such as cyclone separators or the like for particle recovery. The first and second collection devices are in fluid communication with the downstream induced draft fan to draw the driving fluid, the particle mixture, and the particle flows through the plurality of electrostatic separation apparatus and eventually to the appropriate collection device for recovery.
A method of separating two species of particles from a particle mixture is also disclosed. In the broadest aspects of the method, the steps include feeding the particle mixture to a first inlet to create a turbulent particle mixture flow to electrostatically charge the particles, straightening the particle mixture flow, delivering the straightened particle mixture flow to a first separation chamber having a first electric field zone for drawing the selected charged particles therefrom, introducing a smooth curtain gas flow to the separation chamber substantially parallel to the particle mixture flow to entrain and carry the selected charged particles drawn from the particle mixture to a collector for recovery, and delivering the remaining particle flow to a second separator downstream of the first separator. Advantageously, by ensuring that the particle mixture and curtain gas flows remain substantially parallel, turbulence in the separation chamber is minimized, thereby resulting in enhanced separation efficiency.
Still other objects of the present invention will become apparent to those skilled in this art from the following description wherein there is shown and described a preferred embodiment of this invention, simply by way of illustration of one of the modes best suited to carry out the invention. As it will be realized, the invention is capable of other different embodiments and its several details are capable of modification in various, obvious aspects all without departing from the invention. Accordingly, the drawings and descriptions will be regarded as illustrative in nature and not as restrictive.